This invention relates to high power (nonlinear) testing of microwave transistors (DUT). When the transistor is driven in its nonlinear operation regime, the internal impedance at the output of power transistors is very low. An impedance tuner used to match the transistor must also physically match such impedance. Passive impedance tuners can reach maximum reflection factors |Γtuner| of the order of 0.95, corresponding to impedances of 2.4 Ohm at their test port reference plane. The insertion loss between DUT and tuner reduces the available tuning range at the DUT reference plane and thus the capacity of the passive tuner to match the transistor. The only remedy to this limitation is using active systems, i.e. test systems whereby a signal coherent (synchronous) with the signal injected into the transistor, is injected independently into the DUT output terminal and creates a virtual load. This additional signal can be the only one injected, in which case we speak of “active” load pull, or it can be superimposed (vector-added) to signal reflected by a passive tuner, in which case we speak of “hybrid” load pull; obviously if only a tuner is present, we speak of “passive” load pull. In both active injection cases the objective is reaching and matching the conjugate complex internal impedance of the transistor; in general terms a standard requirement is a dynamic range reaching a reflection factor |Γ|=1 (corresponding to an internal impedance real part of 0 Ohm). The objective of this invention is an active tuner apparatus, combining a forward signal injection mechanism within a tuner architecture, allowing |Γ|=1.